The present invention relates to a method of monitoring acid strength during alkylation of an olefinic feedstream by use of at least one refractive index analyzer. In addition, the invention relates to an alkylation process using at least one refractive index analyzer to generate real-time concentration readings. The invention further relates to a method of determining and controlling acid concentration during an alkylation process.
In light of the curtailment in the use of tetraethyl lead as an octane-improving additive for gasoline, the production of unleaded gasoline has increased as well as the octane number specification of all grades of gasoline. Additionally, recent reformulated gasoline specifications require a reduction in both the Reid Vapor Pressure (xe2x80x9cRVPxe2x80x9d) and olefin content. Alkylate is a low vapor pressure, high-octane gasoline blending component containing essentially no olefins. Thus, alkylate helps refiners meet the new reduced RVP and reduced olefin content specifications. Additionally, alkylate burns cleanly, resulting in lower levels of undesired emissions from gasoline engines.
Alkylation, a well-known refinery process for converting light, gaseous olefins into high-octane gasoline components, involves the addition of an alkyl group to an organic molecule. In alkylation, an isoparaffin is typically reacted with an olefinic hydrocarbon feed to provide an isoparaffin of higher molecular weight. Generally, the alkylation of isoparaffins with olefins is accomplished by contacting the reactants with an acid catalyst such as hydrogen fluoride or sulfuric acid, settling the mixture to separate the catalyst from hydrocarbons, and further separating the hydrocarbons, usually by fractionation to recover the alkylated product. The alkylation reaction product is referred to as xe2x80x9calkylatexe2x80x9d, and it preferably contains (in order to render the highest quality gasoline blending stock) branched chain hydrocarbons having five to sixteen carbon atoms, with the exact composition depending upon the isoparaffin and olefinic hydrocarbon feed used, as well as process conditions.
The olefinic hydrocarbon feed generally comes from a catalytic cracker and contains olefins, paraffins, and isoparaffins in the C3-C5 range. Common impurities present in the feed are mercaptan sulfur, diolefins, and free water. Diolefins, such as butadiene, present in the olefinic hydrocarbon feedstream, are known to consume the acid catalyst at rapid rates. The result is the formation of acid-soluble hydrocarbons, known as red oil or acid-soluble oil (ASO) in the acid phase and a lowering of the quality of the alkylate octane.
Under optimum conditions in commercial alkylation reactors, sulfuric acid usually enters the reactor at 98% weight strength and exits at 89% weight strength. Each percent above the 89% target that the acid exits the reactor represents a 10% waste in total acid consumption. In the alkylation of isoparaffins and olefins with a strong mineral acid such as sulfuric acid, it is critically important to be able to recycle the used or spent acid back to the reactor. This used or spent acid is comprised of three componentsxe2x80x94acid, water, and red oil or ASO. The latter accumulates in the acid phase, thereby lowering the acid strength of the catalyst. The composition of red oil in an alkylation unit varies depending upon the feed composition and reaction conditions. Red oil is soluble in the acid catalyst and may be chemically bound by the strong acid catalyst. It is important to know the acid content of the recycle acid in order to determine the amount of fresh acid needed to bring the mixture of fresh and recycle acid to the desired concentration of acid in the alkylation reactor.
If the acid strength within the alkylation reactor falls below about 86%, xe2x80x9cacid runawayxe2x80x9d becomes eminent, where the acid strength depletes so rapidly that the feedstock into the unit must be cut off. It is then necessary to increase the flow of fresh acid in order to halt the degradation. If an unusually rapid drop in acidity is detected before the acidity drops below the safe minimum acidity, the acidity can usually be brought back to a safe level by increasing the fresh acid feed.
Previously, operators relied on chemical laboratories for acid titration data to determine acid catalyst strength. In so doing, they were gambling on the possibility that the acidity could not be raised to a point at which the acid could act as catalyst. Furthermore, such methods were not responsive to short-swing upsets during operations.
Several attempts have been made to measure acid strength on physical properties of the catalyst. For example, U.S. Pat. No. 3,653,835 teaches measuring the specific gravity of a sample of spent sulfuric acid as a means of measuring the concentration of acid. U.S. Pat. No. 3,935,097 describes a system directed to high-pressure liquid chromatography for separation of acid and water. Further U.S. Pat. No. 4,009,998 discloses a method for measuring the concentration of acid by electrical conductivity. Still further, some operators have used the viscosity of the spent acid to correlate the acidity of the system acid. Such methods have seen limited success primarily because they use an indirect means to correlate the acidity. Furthermore, the presence of red oil in varying amounts can adversely influence the measurement. All of these methods are based on sampling and do not offer the alkylation plant operator the ability to maintain a continuous control of the acidity of the alkylation catalyst and thus control of the quality of product.
Continuous on-line analysis of acid strength by near-infrared spectrophotometry is disclosed in U.S. Pat. No. 5,681,749. Such means, however, requires advanced training and is relatively expensive to maintain. Other methods have been employed in an attempt to achieve in-situ determination of the acidity of acid-water solutions. For example, on-line continuous acidity analysis has been documented by monitoring velocity of sound in the flowing acid stream. This system is dependent upon the density of the medium and is accurate only for certain acid-water solutions that do not contain red oil. Other techniques have been utilized to measure on-line acidity strength include nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d).
All of these techniques present serious limitations including limited accuracy, complex modeling, and sample conditioning requirements contributing to application complexity, high installation costs, and maintenance/reliability concerns. In addition, they fall short of meeting an operator""s need to accurately monitor and control the acid strength in commercial operations. In commercial plants, the amount of red oil content of acid typically varies over a wide range. Thus, it is desirable to have a method that can reliably measure the acid strength regardless of the variations in the red oil content.
It is an object of this invention to provide a method for the alkylation operator to continuously monitor on-line and control with confidence the acid strength in a commercial hydrocarbon conversion process.
In particular, it is an object of the invention to provide a method for use by the alkylation plant operator to adequately maintain continuous control of the acidity of the alkylation catalyst and therefore control the product octane quality.
It is further an object of this invention to provide a reliable method of measuring the acid strength in a mixture comprising a mineral acid, water and red oil in a hydrocarbon conversion process by a continuous in-line technique to enable operators to make adjustments to their fresh acid addition rate and spent acid purging rate, thereby improving product quality.
It is also an object of this invention to provide a method for determining the concentration of acid by an on-line analyzer which affords greater accuracy and which is easier to use than the on-line analyzers of the prior art.
The invention relates to an alkylation process employing a refractive index analyzer to monitor, control and/or determine acid catalyst strength during alkylation of an olefinic feedstream. In a preferred embodiment, the invention relates to the alkylation of a hydrocarbon mixture comprising olefins and paraffins with a sulfuric acid catalyst. The acid enters the akylation reactor train at approximately 98% weight strength and exits at approximately 89% weight strength. The concentration of acid is controlled and maintained by monitoring the refractive index of the acid in the product mixture, most preferably comprising mineral acid, water and red oil. Online analyzer results may be compared to the results of manual laboratory tests on the acid strength of the catalyst using manual sample analyses or titration methods. Periodically, after calibration of the system, samples may be taken to verify the precision of the online analyzer, if desired.
The method of the invention permits a determination of alkylation catalyst with a precision that is comparable to that which can be achieved with large bench-top, non-portable instruments. Moreover, in a preferred embodiment, the present invention may further provide a direct readout display of the acid concentration.
The refractive index analyzer for use in the method of the invention includes a refractive index sensor with a refractometer prism having a measuring surface which contacts the outer surface of pipe or similar conduit through which passes the product mixture of alkylate, mineral acid, water and red oil. Furthermore, the sensor preferably includes a substantially monochromatic light source disposed with the sensor such that the light source, upon activation, is capable of directing a light beam through the conduit or is redirected via at least one mirror. In so doing, light enters the light entrance side of the prism and at least a portion of the light is refracted from the measuring surface of the prism into the sample medium and at least a portion of the light is reflected back through the exit side of the prism to a photodetector. The portion of beam reflected to the photodetector is dependent on the boundary formed by critical angle "PHgr"CRIT at the measuring surface. This angle is dependent on refractive indices of the prism and the liquid in contact with the measuring surface. The refractive index of the mixture in contact with the measuring surface is dependent on the percentage concentration of alkylation catalyst in the liquid and is collected by an image detector and image digitizer capable of generating a xe2x80x9cdrift-freexe2x80x9d digital image signal that may be displayed and/or processed after being transmitted to at least one processor by at least one transmitter. Ultimately, the percentage concentration of the constituent may be visually displayed and monitored.
The sensor, may further be used in conjunction with other sensors that are linked to transmitters such that the concentration of acid in the product mixture may be monitored at least one location within a plant, refinery, or similar structure.
In a preferred embodiment, the alkylation catalyst is sulfuric acid in which the sensor and transmitter provides information related to the concentration of the sulfuric acid within the hydrocarbon mixture such that the concentration of sulfuric acid may be monitored and maintained at the requisite level.